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So l am changing a dented rocker cover on my engine in the shop. Figured out the ext. Manifolds had to come off. Pull the risers and see rust in the bottom of the manifold Go to pull the manifold and I find the rear bolt broken the next 2 are bent ??In addition to your cylinder head castings being "light castings", you will want to examine the threaded area where the manifold bolt became bent! Look to see if it cracked the area around the female threads!

When the manifold comes off there's rust in the ports @%$$. Off comes the head and there's water in 1 hole and rust in another.

So when the tree fell on the boat I got this engine from (see post " I killed a SeaRay") it knocked the the manifold loose. Rain or snow melt got into the engine and has set for a couple of years. Like I said sometimes it sucks to be meContact Witch Rustucia...... she can help with the rust!

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Mike the back bolt snapped as soon as I put torque on the ratchet. Way before I would have thought the bolt would have come loose and long before it should have broken. The next bolt felt strange when undone and once out I could see the bend in it. The next 2 came out normal. Once the manifold was off I unscrewed the piece still in the head with my fingrrs

1992 2452
5.0 alpha1 gen2

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Mike the back bolt snapped as soon as I put torque on the ratchet. Way before I would have thought the bolt would have come loose and long before it should have broken. The next bolt felt strange when undone and once out I could see the bend in it. The next 2 came out normal. Once the manifold was off I unscrewed the piece still in the head with my fingrrs

So the back bolt broke immediately yet the remaining piece unscrewed by hand? How to the threads in the head look?

1997 Maxum 2400 SCR 5.7LX Bravo II

Mike

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All of the bolts required a wrench, the back bolt broke as soon as I tried to turn it. The next 3 were not as tight as they should have been but did require a wrench. What I could remove with my fingers was the stub of the broken bolt once the manifold was off.There was a small bit of the bolt sticking out of the head . I was able I will have to look at the threaded holes in the head after work or this weekend. If they look odd I will get a pic up

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So the back bolt broke immediately yet the remaining piece unscrewed by hand? How to the threads in the head look?

This is not unusual. I've the same thing happen a number of times when I was working on vintage motorcycles. I believe that the tension load on the bolt, along with some rust in the threads locks it in place. The sudden release of the tension and vibrations from the bolt breakage loosens everything up, allowing the remaining piece to be easily removed.

This is not unusual. I've the same thing happen a number of times when I was working on vintage motorcycles. I believe that the tension load on the bolt, along with some rust in the threads locks it in place. The sudden release of the tension and vibrations from the bolt breakage loosens everything up, allowing the remaining piece to be easily removed.

Agree but he said it broke immediately so I would have expected the piece to be recessed in the hole and not where he could get his fingers on it to screw it out. Also it’s just a small portion of the threaded section so if it broke immediately it would be in the hole unless the threads got pulled from the tree hitting the manifold.

1997 Maxum 2400 SCR 5.7LX Bravo II

Mike

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That small piece of bolt is about all that's threaded into the head on any of the holes I looked at some of the other exhaust bolts that's about as far as any go in.
As for the threads in the head, I ran a bolt into that hole and it feels the same as all the others

I took the pan off today.
What are we paying for in a Mercury Motor?? I would think that the MX6.2 was Mercury's HOT ROD small block.
What I have found is a stock GM 2 bolt main block, light casting heads, stamped rockers. Outside of the stroked crank, better rods and pistons this is your average Malibu / Tahoe motor!!
I know the freeze plugs are different. All the bolt on stuff has j1171 marine stickers on it. They tweaked the F.I. to work in a marine environment (ie no return to the tank).
Where are the hot rod parts? Deburred and clearanced block, 4 bolt mains, studded block, roller rockers, rocker stud girdle, These motors spend their life at 3000 RPM + where are the parts to back that up?
I'm not a marine mechanic so I guess I just have to believe this is what you get for your boat motor bucks ?? I would love to see some failure analysis history from Merc. What brakes first?
What brakes most often?
OK I will get off my soapbox. I know enough to know, I don't know what I'm talking about here!

What I did find when the pan came off.
Water in everything.
I didn't see where it had gotten to the crank. It pushed oil up to above the windage tray. The crank rolls normally. I pulled #3 and #7
pistons. The rings are loose on both pistons. Ran a hone down both holes and still don't much like what I see. Not sure you can see it in the pix #7 still has a bit of a "feel" to it. The #3 hole is just discolored. ​​​​​​​
I have some pix of the #7 rod bearing it looks to me like it was not long for this world. ​​​​​​​ I don't know why the wrist pin side of the #7 rod looks discolored? ​​​​​​​
SO what to do from here?? I think I'm going to take the block all the way down tomorrow. Bore and replace the pistons and bearings? I'm hoping the cam looks good, I'd rather not replace it and the lifters. Or keep the 3.75 stroke crank. Look for a 4 bolt main block. Have it machined up to run 383 ci. Deburred and clearanced , a set of ARP bolts and studs, roller rockers..............never mind I haven't won the lottery yet. Somebody convince me all the stuff I learned drag racing about high RPM, high horsepower small blocks does not apply to boat motors and I should just put this back together and get out on the water in this lifetime

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You can have the third and fourth bolts added to the block. Merc just buys sorta heavy duty long blocks and builds from there. The really good motors come from specialty builders. I'm suspect of mass engine rebuilders as a rule, but others have recommended some companies.
As to what you have, it looks like the previous owner was hard on the engine and maybe low on maintenance. I've seen and successfully rebuilt worse. Don't get caught up in gee whiz names of brands, use boring tried and true parts, but, assemble them as though they are going to be a hard working system, because they are.

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IMO you’ve discovered enough and have torn it down enough to warrant having the block and crank machined at the very least. New bearings throughout and new QE pistons. With that said If you’d like to go to a 4 bolt this is the time to do it but not sure that’s necessary for the rpm range these boats typically cruise at.
You’ll be spending some money, Just for kicks take a look at this. https://www.summitracing.com/parts/mll-bp3834

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I took the pan off today.
What are we paying for in a Mercury Motor?? I would think that the MX6.2 was Mercury's HOT ROD small block.We don't want a Hot Rod engine in a cruiser.

What I have found is a stock GM 2 bolt main block, light casting heads, stamped rockers. Outside of the stroked crank, better rods and pistons this is your average Malibu / Tahoe motor!!

I know the freeze plugs are different.

All the bolt on stuff has j1171 marine stickers on it. They tweaked the F.I. to work in a marine environment (ie no return to the tank).Correct..... mapping is quite different and a return fuel line is not used.

Where are the hot rod parts? Deburred and clearanced block, 4 bolt mains, studded block, roller rockers, rocker stud girdle, These motors spend their life at 3000 RPM + where are the parts to back that up?While the engineers may know what should be done, the GM bean counters are given the last word.

I'm not a marine mechanic so I guess I just have to believe this is what you get for your boat motor bucks ??Yep, unless you build your own custom SBC Marine engine.

I would love to see some failure analysis history from Merc. What brakes first?
What brakes most often?
OK I will get off my soapbox. I know enough to know, I don't know what I'm talking about here!I understand and can appreciate your disappointment.

What I did find when the pan came off. Water in everything.

I didn't see where it had gotten to the crank. It pushed oil up to above the windage tray. The crank rolls normally. I pulled #3 and #7
pistons. The rings are loose on both pistons. Ran a hone down both holes and still don't much like what I see.
Not sure you can see it in the pix #7 still has a bit of a "feel" to it. The #3 hole is just discolored.

I have some pix of the #7 rod bearing it looks to me like it was not long for this world.Not good!

I don't know why the wrist pin side of the #7 rod looks discolored?It would help to see the piston deck, the surrounding piston decks and the corresponding combustion chambers.
I'm curious as to which piston profile was used.... please post a few photos.

​​​​​​​

SO what to do from here?? I think I'm going to take the block all the way down tomorrow. Bore and replace the pistons and bearings?

I'm hoping the cam looks good, I'd rather not replace it and the lifters.SBC roller camshafts and roller cam followers will go for miles and miles..... I'd be surprised if yours were not re-usable.
FYI, if you were to replace it, the camshaft must incorporate a small base circle for clearance reasons.

Or keep the 3.75 stroke crank. Look for a 4 bolt main block. Have it machined up to run 383 ci. Deburred and clearanced , a set of ARP bolts and studs, roller rockers..............never mind I haven't won the lottery yet. Somebody convince me all the stuff I learned drag racing about high RPM, high horsepower small blocks does not apply to boat motors and I should just put this back together and get out on the water in this lifetime

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IMO I would start with a none marine block as the water jackets in this one have been getting thinner over the years as scale falls off. I would then add closed cooling. As Rick stated you don’t want a hot rod engine for this boat. Too much overlap in the cam increase potential for water ingestion.

1997 Maxum 2400 SCR 5.7LX Bravo II

Mike

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I agree that you don't want a hot rod motor. However, you do want a strong easy running purpose built motor. That's why I advocate balancing and blueprinting the engine. The easier the rotating machinery can spin the less "work" it has to do. Just my opinion, but like others are pointing out, there's a difference between adiquate and totally correct. Manufacturers largely build to adiquate, unless they are building specialty hardware, because, in reality, most customers don't really know or care about the difference. Even those "high end" ski boats that cost huge bucks use "off the shelf" power packages that could just have easily been sent to Bayliner or SeaRay for installation.